breeding for 2n egg production in haploid × species 2x potato hybrids

1991) 691

BREEDING F O R 2N EGG P R O D U C T I O N IN H A P L O I D X SPECIES 2X POTATO H Y B R I D S

Rodomiro Ortiz and S. J. Peloquin 1

Abstract

The identification of diploid (2x), Tuberosum haploid x wild species hybrids (HS) which produce high frequencies of 2n eggs and have good tuber appearance is important for obtaining 4x progeny from 2x x 2x crosses in potato. More than 1700 HS clones were screened for the occurrence and frequency of 2n eggs through 2x x 4x crosses during two pollination periods. The resulting seed/fruit provides an estimate of 2n egg frequency. About 650 HS clones produced 2n eggs; 63 had 10-114 seeds/fruit; and 27 were selected for good tuber type. There was a large variation in seed set after 2x x 4x crosses, both within and between clones. Compari- son of fruit and seed set between the two pollination periods using the same 595 clones indicated that environmental factor significantly affected 2n egg frequency. The mode of 2n egg formation was mainly due to omission of the second meiotic division controlled by a recessive gene (os). The gene frequency for os varied from 0.28 to 0.76 among six, 2x taxa. Both additive and dominance variance were significant for seed set after 2x x 4x crosses. Narrow sense heritability was estimated as 0.24. The average degree of dominance was equal to 1.66 which could indicate overdominance. Alternatively, this could be due to linkage disequilibrium, i.e. pseudo- overdominance. However, dominance variance was greater than the additive variance indicating recurrent selection can be useful for improving the frequency of 2n egg production in the 2x population. The selected individuals, based on their phenotypic performance, should be progeny tested to identify the best parents for use in the next cycle of recombination.

Compendio

La identificacion de hibridos entre haploides de Tuberosum (2n -- 2x -- 24 cromosomas) y especies silvestres (HS) que producen altas fre- quencias de oosferas 2n y tienen aceptables atributos agronomicos es im- portante en la obtencion de progenies 4x de cruzamientos 2x x 2x. Mas de 1700 clones HS fueron evaluados a traves de cruzamientos 2x x 4x. Los resultados de tales cruzamientos proveen un estimado de la frequencia de oosferas 2n. Cerca de 650 clones HS produjeron oosferas 2n; 63 tuvie-

1Departments of Horticulture and Genetics. University of Wisconsin, Madison, W153706. Accepted for publication February 16, 1991. ADDITIONAL KEY WORDS: 2x x 4x crosses, omission of second meiotic division, recurrent selection.

692 AMERICAN POTATO JOURNAL (Vol. 68

ron de 10 a 114 semillas/fruto; y 27 fueron seleccionados por sus atributos agronomicos. Hubo una gran variacion en semillas por fruto despues de cruzamientos 2x x 4x, tanto entre y dentro de clones. La comparacion de produceion de frutos y semillas por fruto entre dos periodos de poliniza- cion, utilizando los mismos 595 clones, indico que el ambiente afecto sig- nificativamente la frequencia de oosferas 2n. Oosferas 2n fueron formadas principalmente debido a la omision de la segunda division meiotica, que es controlada por un gen recesivo (os). La frequencia del gen os vario de 0.28 a 0.76 entre seis especies diploides. Tanto la variancias aditiva como de dominancia fueron significativas para la produccion de semillas por fruto despues de cruzamientos 2x x 4x. La heredabilidad estimada rue 0.24. E1 grado de dominancia fue calculado como 1.66, indicando sobredominan- cia. Esto puede deberse a desequilibrio de ligamiento, en efecto pseu- dosobredominancia. A pesar de que la variancia de dominancia fue mayor que la aditiva, seleccion recurrente puede aplicarse para acrecentar la fre- cuencia en la produccion de oosferas 2n. Sin embargo, los individuos seleccionados de acuerdo a su expresion fenotipica, deben ser evaluados de acuerdo a una prueba de progenie para ser incluidos en el siguiente ciclo de recombinacion.

Introduction

The incorporation of important traits from wild Solarium species into the cultivated 4x potato can be achieved through the use of haploid x species hybrids which produce 2n gametes (gametes with the sporophytic chromosome number). They can be used either in 4x x 2x, 2x x 4x or 2x x 2x crosses to produce 4x progenies with the desirable attributes (6). The

final utilization of such products could be in both variety development or potato production from true seed (TPS).

Cytological observations permit the detection of the mode of 2n egg formation (2, 10, 12). Werner (13), using the staining clearing technique (9) found that 2n eggs in potato are mainly formed through omission of second meiotic division (genetically equivalent to an SDR mechanism). Omission of the second meiotic division is genetically controlled by the recessive mutant "os" (I4).

Seed set following 2x x 4x crosses provides an indication of 2n egg production in the 2x female. Werner and Peloquin (12) indicated that there was a significant correlation between seed set per fruit after 2x x 4x crosses and the cytologically determined frequency of 2n eggs. This means that the screening for 2n egg production can be done by 2x x 4x crosses.

The objectives of the present research were to: a) evaluate the production of 2n eggs in a haploid x species hybrid population; b) identify 2x Tuberosum haploid x wild species (HS) hybrids which produce a high frequency of 2n eggs and have good tuber appearance; c) determine the type

1991) ORTIZ AND PELOQUIN: BREEDING FOR 2N EGG PRODUCTION 693

and amount of gene action controlling seed set after 2x x 4x crosses; and d) develop a breeding procedure to increase the frequency of 2n egg production based on seed set after 2x x 4x crosses in the 2x population.

Material and Methods

A total of 1361 clones representing 129 haploid-species hybrid families were planted at Rhinelander during the summer of 1989. The haploid parents were extracted mainly from the 4x clones W-231 and Atlantic which are both simplex (Osososos) for the os locus (13). The wild species were S. bu~ovii, S. ehacoense, S. sparsipilum and natural hybrids between S. berthaultii x S. tarije~e. Another 385 clones selected from three different breeding

populations were also planted: a) 262 HS clones from a population which is undergoing recurrent selection for 2n pollen; b) 59 H S clones selected for both 2n pollen and good tuber appearance from crosses between haploids and 10 different wild species, and c) 64 clones from CIP's 2x breeding population, mainly with S. tuberosum gp. Phurqa and gp. Stenotomum, which had been selected for tuberization under long-day conditions and for 2n pollen production.

Each done which flowered was pollinated following emasculation with pollen of one of the following 4x: W-231, W-760, W-877, ND-860-2, or Rar- itan, during the first pollination period (07/12/89 to 08/09/89); and with W-856, W-877, W-887 or Raritan during the second poUination period (08/16/89 to 08/24/89). Atlantic and W-1061 were used as females in control crosses with 4x males. Fruits, from successful crosses, were wrapped in cheesecloth to prevent loss and harvested a minimum of 4 weeks post~ pollination. Fruits were allowed to mature for another three weeks and seeds were extracted from each fruit separately. The number of seeds/fruit provided a measure of 2n egg frequency in each done.

Statistical Analysis

The haploid parents H-551, W-730 and W-973 (extracted from W-231) were used as testers in crosses with the wild species Plant Introductions (PI). The three clones are homozygous for the os locus (13). The genotypes of the other haploids were determined using the wild species Prs with known genotypes as testers.

X 2 tests were carried out to test if the observed ratios fit the binomial distribution assuming Os/os as the genotype of the PIs. The expected ra- tio of the cross Os/os x os/os is 1:1 of non 2n to 2n egg producing individuals. Thus, the X 2 test was used for testing for homogeneity to a binomial distribution with p -- 0.5 and q = 0.5. For any sample of N size, the minimum number of non 2n egg producers observed in a sample should be more than Np-2(Npq) 1/2 with 95% confidence. Otherwise, the cross is not of the Os/os x os/os type. If the non 2n eggs producers observed are less than


the expected minimum, the cross may be of os/os x os/os type if there is incomplete penetrance or misclassification. In general, the observed number of non 2n (or 2n, if this is the smaller observed number) individuals should be higher than the expected minimum from the binomial distribution if the assignment of the genotypes is correct.

The quantitative variation for seed set (over both pollination periods) between and within 19 families was evaluated. The families were derived from crosses between 2n egg producers (os/os) using 4 haploid parents: W- 730, W-973, A-401 and H-551, with the wild species S. bukasovii, S. chacoense, and S. s ~ . The analysis was carried out using a nested design in which individuals were nested in families, and families in females. Each family consisted of a variable number of individuals with a maximum of 22. Each individual had a different number of pollinations and fruit set with a maximum of 11 fruit/individual. The statistical analysis for hierarchical design combined over both pollination periods was performed using SAS (7). The analysis of variance was performed using GLM procedure and the esti- mation of components of variance by VARCOMP procedure.

The components of variance provided estimates of environmental variance, and additive and dominance variance (1). This allowed the estima- tion of narrow sense heritability (h 2) and degree of dominance ("a') for quantitative variation in seed set. These statistics were used for the esti- mation of gain from selection in seed set after 2x x 4x crosses following recurrent selection in the 2x breeding population (1). Gains per cycle after selection using different intensities of selection, number of environments and pollinations were calculated.

Results and Discussion

A total of 7034 and 4498 pollinations were done during the first and second pollination periods, respectively. The percentage of fruit set was 20.8 % during the first pollination period and 36.3 % during the second. However, an increase of parthenocarpic fruit development was observed during the latter. Also 275 and 350 pollinations were done with 48 selected haploid-species hybrids and 58 clones from CIP's 2x breeding population, respectively.

Twenty-seven clones from crosses between Tuberosum-haploids x species and five clones from the recurrent selection program for 2n pollen had both a high frequency of 2n eggs and good tuber appearance (Table 1).

Variability for seed set in both 4x x 4x and 2x x 4x crosses

A large variability betweeen clones for seed set was observed in both pollination periods. The number of seeds per fruit after 2x x 4x crosses varied from 0 to 113 during the first pollination period and from 0 to 114 during the second pollination period. Seed set in 4x x 4x crosses varied


TABLE 1.--Pech'grees, number of pollinations and fruits, and seed set after 2x x 4x crosses of selected 2x clones for both 2n eggs production and tuber type along with those of 4x families after 4x x 4x crosses.

Code Pedigree No. poll. No. fruits Seed/Fruit with seeds . . . . . . . . . . . . . . . . . . . . . . .

M e a n Range

1. 2n eggs producers: a) haploid species hybrids: DH-180 W-730 x ver 7 4 10.0 4-17 DH-183 W-1887 x grl 5 3 11.0 6-19 E-186 W-730 x spl 19.5.50 12 1 10.0 E-205 W-730 x spl 19.81.17 3 2 18.0 9-27 E-206 W-730 x sp119.81.17 7 3 9.7 1-25 E-208 W-730 x spl 19.81.17 5 3 15.3 7-23 E-503 W-973 x buk 20.9.52 4 2 17.5 7-28 E-523 W-973 x buk 20.17.70 8 2 29.5 18-41 E-546 A-482 x b u k A 9 14 6 /6.8 2-39 E-641 H-551 x spl 19.5.50 9 6 21.0 11-60 E-643 H-551 x sp119.5.50 10 2 11.5 5-18 E-649 H-551 x spl 19.5.50 7 1 59.0 E-650 H-551 x spl 19.5.50 7 2 12.5 1-24 E-666 H-551 x spl 19.22.56 9 1 16.0 E-669 H-551 • spl 19.22.56 6 1 14.0 E-886 H-551 x chc 23 6 4 26.2 18-35 E-968 W-730 x tar 6 1 38.0 E-1003 A-482 x (ber x tar) 5 1 114.0 E-1041 H-556 x (bet x tar) 7 2 18.0 16-20 E-I073 A-482 x (ber x tar) 3 1 46.0 E-1077 A-482 x (ber x tar) 11 7 25.4 1-70 E-1084 A-482 x (her x tar) 8 1 10.0 E-1106 H-556 x (ber x tar) 9 5 11.2 3-18 E-1107 H-556 x (ber x tar) 5 2 10.0 7-13 E-1122 H-551 x (ber x tar) 2 1 14.0 E-1130 W-730 x (ber x tar) 7 1 37.0 E-1269 A-241 x chc23 7 1 15.0 b) Recur ren t selection for 2n pollen: E-1397 (W-730 x mlt) x

(W-730 x ver) 10 7 63.7 45-92 E-1399 (W-730 x mlt) x

(W-730 x ver) 6 5 67.2 1-259 E-1401 (W-730 x mlt) x

(W-730 x ver) 6 5 21.0 20-22 E-1542 (W-1887 x vrn) x

(W-973 x grl) I1 5 12.5 8-17 E-1545 (W-1887 x vrn) •

(W-973 x grl) 5 2 15.5 3-28 2. 4x parents: Atlantic 26 16 73.4 11-176 W-1061R 19 12 106.1 30-282 Mer r imack 7 5 78.8 50-111


from 30 to 152 seeds per fruit during the first pollination period and 61 to 96 seeds per fruit during the second pollination period. Variation for seed set was also found within each cross in both 2x x 4x and 4x x 4x crosses (Table 1). This indicates that seed set in potato is a complex process of reproductive biology, in which both environment and genotype of the parents have significant effects on seed/fruit.

Both Genetic Background and Environment Affect Production of 2n Eggs

The results indicating the number of non 2n and 2n egg producers in each family using the haploids W-730, W-973 and H-551 as female parents are listed in Table 2. The fact that few individuals from families from crosses between 2n egg producers did not produce seeds is an indication that seed set following 2x x 4x crosses only measures the relative frequency of 2n eggs. For example, individuals from families 271, 272, 273, 291, 294, 297, 318, 332, 342, 346 (Table 2), with a low frequency of 2n eggs could be misclassified as non 2n egg producers.

There were some families, involving S. sparsipilum as the male parent, which did not fit the expected ratios, e.g., 278, 280, 281, 324, 326, and 327. There were more non 2n egg producers than expected. One possible ex- planation could be the lack of penetrance of the os gene in such hybrids with S. sparsipilum. Alternatively, an excess of 2n egg producers was observed in families in which the species parent was S. bukasovii, e.g. families 290, 330 and 333. Likewise, an unexpected success of seed set after 2x x 4x crosses was found in single individuals of families whose parents were Tuberosum haploids such as H-556, H-373 and H-322 with wild species having Os/Os genotypes (data not shown). These could result from other mechanisms of 2n egg formation different than omission of the second meiotic division.

Pollinations were made twice with a group of 595 clones (Table 3). The results indicate that the environment is important in fruit and seed set; i.e. a higher proportion of 2n egg producers with higher frequencies of 2n eggs was found in the second pollination period. The second pollination period was characterized by a lower temperature than the first pollination period. The lower temperature also resulted in an increase of fruit set, sometimes as a result of the development of parthenocarpic fruits. Thus, the lower temperatures could allow the retention of fruits with one or two seeds/fruit.

Another 597 clones were pollinated only once during the season. The results (Table 4) indicate that late flowering types had more 2n egg producers than early flowering types. However, it was possible to identify early flowering types with high 2n egg frequencies.

Frequency of "os" Gene m 2x Solanum Species

The fact that 2n egg producers were found in almost all the Tubero- sum haploids x species hybrid families and in other breeding populations


TABLE 2.--Pe&'grees, genotypes, non 2n egg and 2n egg producers, and expected ratios in Tuberosum haploids (os/os) • species families.

Family Haploid Species Genotype Observed Expected (PI) of the . . . . . . . . . . . . . Minimum1

Species non-2n:2n non-2n or (2n)

271 W 730 buk A8 os/os 2 11 2.97 272 W 730 buk A9 os/os 5 13 318 H 551 buk A9 os/os 1 11 9.642 273 W 730 buk B os/os 4 19 6.80 275 W 730 buk B4 Os/os 6 12 4.84 278 W 730 spl 113.2 Os/os 16 6 (6.40)* 279 W 730 spl 113.5 Os/os 6 5 (2.18) 280 W 730 spl 113.8 Os/os 17 2 (5.23)* 281 W 730 spl 113.15 Os/os 11 2 (2.97)* 282 W 730 spl 19.5.50 Os/os 15 7 302 W 973 spl 19.5.50 Os/os 4 6 322 H 551 spl 19.5.50 Os/os 4 11 16.782 284 W 730 spl 198177 Os/os 10 7 (4.46) 285 W 730 spl 199102 Os/os 5 9 3.33 288 W 730 buk 19.31.4 Os/os 8 16 7.20 289 W 730 buk 20.9.47 Os/os 8 11 330 H 551 buk 20.9.47 Os/os 2 9 9.632 290 W 730 buk 20.9.52 Os/os 2 11 307 W 973 buk 20.9.52 Os/os 2 7 6.40* 291 W 730 buk 201065 os/os 2 9 308 W 973 buk 201065 os/os 3 8 332 H 551 buk 201065 os/os I 5 8.812 292 W 730 buk 201079 Os/os 6 9 3.80 293 W 730 buk 201648 Os/os 4 9 2.97 294 W 730 buk 201654 os/os 1 8 335 H 551 buk 201654 os/os 0 6 3.80 295 W 730 buk 201763 Os/os 7 9 336 H 551 buk 201763 Os/os 4 7 8.30 296 W 730 buk 201770 Os/os 7 16 310 W 973 buk 201770 Os/os 2 7 337 H 551 buk 201770 Os/os 4 7 14.942* 502 W 730 ber-tar 1.33.8 Os/Os 10 0 511 H 551 ber-tar 1.33.8 Os/Os 15 0 (7.50) 2 520 W 730 ber-tar 1.13.5 Os/os 5 4 (1.56) 525 W 730 ber-tar 3.15.10 Os/Os 7 0 (0.85) 297 W 973 buk A os/os 1 8 1.56 317 H 551 buk A3 Os/os 11 6 (4.38) 321 H 551 spl C3 Os/Os 21 2 (6.80)

xIfthe expected minimum was higher than observed, then Os/os genotype for the PI parent was rejected. 2Expected minimum pooled over the PI parent. *Deviated significantly from expected outcome assuming Os/os genotype for the PI. buk = S. bukasovii chc = S. chacoense spl = S. sparsipilum beT-tar = natural hybrid between S. berthaultii x S. tarijense.

698 AMERICAN POTATO JOURNAL

TABL~ 2.-- continued

(Voi. 68

Family Haploid Species Genotype Observed Expected (PI) of the . . . . . . . . . . . . . Minimum1

Species non-2n:2n non-2n or (2n)

324 H 551 spl 19.9.04 Os/os 9 1 (1.84)* 325 H 551 spl 192250 Os/os 14 6 (5.53) 326 H 551 spl 191931 Os/os 13 0 (2.97)* 327 H 551 spl 192828 Os/os 19 4 (6.70)* 328 H 551 spl 192919 Os/os 4 9 2.97 333 H 551 buk 201279 Os/os 4 16 5.53* 342 H 551 chc 23 os/os 2 14 4.00 343 H 551 chc 24 Os/os 7 8 3.63 344 H 551 chc 26 Os/os 8 6 3.25 345 H 551 chc 27 Os/os 7 7 3.33 346 H 551 buk A1 Os/os 1 5 0.55 510 H 551 ber-tar 3.16.1 Os/os 7 3 (1.84)

qfthe expected minimum was higher than observed, then Os/os genotype for the PI parent was rejected. 2Expected minimum pooled over the PI parent. *Deviated significantly from expected outcome assuming Os/os genotype for the PI. buk = S. bukasovii chc = S. dmcoense

spl = S. sparsipilum ber-tar = natural hybrid between S. berthaultii x S. taro'ense.

TABLE 3.--Screening for 2n eggs using 595 clones pollinated during two dzfferent periods at Rk'ndander

Date n eggs 2n eggs Total # of poll. Seed/E (average) (range)

07/12/89 to 418(70%) 177 (30%) 595 5.5

08109/89 0-71

08/16/89 to 333 (56%)

08/24/89 262 (44%) 595 6.3 0-114

x 2 " 26.078"*

TmLE 4.--&'teem'rig for 2n eggs between early and late flowering types.

Flowering type n eggs 2n eggs Total Seeds/Ft.(range)

Early clones 325 (65%) 175 (35%) 500 0-113

Late dones 47 (43 %) 50 (52 %) 97 0-31.25

x 2 - 9.557**


(data not shown) indicates the widespread occurrence of this meiotic modifi- cation in 2x potatoes. The frequency of"os" in several Solarium species was estimated using the proposed genotypes of the wild species parents, based on the segregation ratios for 2n vs non 2n eggs producers in haploid-species hybrid families, since the clones W-730 and W-973 are known to be os/os (13) and only produce 2n eggs by omission of the second division. The estimated gene frequencies were: 0.76 for S. bukasovff, 0.60 for S. chacoense, 0.44 for S. sparsipilum, and 0.28 for the natural hybrid population S. berthaultii x S. tarijense. Using a similar approach the gene frequency of %s" was estimated to be 0.70 for the Tuberosum haploid population. The fact that 2n eggs were also found in clones involving other wild species S. gourlayii, S. infund~iforme, S. nndtidissectum, S. phurqa x S. stenotomun, S. vernei, and S. ver- rucosum, indicates the widespread occurrence of 2n eggs among the cultivated and wild 2x potatoes.

The most common mechanism of 2n pollen production is due to par- allel spindles during second meiotic division (3), which is controlled by the recessive meiotic mutant ps (4). Watanabe and Peloquin (11) reported high frequencies of"ps" in the cultivated groups of potato and their related wild species. Therefore, the high frequency of both os and ps genes in 2x wild and cultivated potatoes supports the hypothesis that bilateral sexual polyploidization, Le. 4x progeny from crosses between parents forming 2n eggs by omission of the second division with parents that form 2n pollen by par- allel spindles, has been involved in the origin of the 4x cultivated potato.

Quantitative Variation for Seed Set After 2x x 4x Crosses

Variation in seed set following 2x x 4x crosses both within and among clones (genotypes) indicates large variability in the frequency of 2n eggs in diploid potatoes and appears to be affected by both the genotype and the environment. However, the analysis of variance for seed set after 4x x 2x crosses during the two pollination periods indicates only significant

differences between genotypes within families (Table 5). All other sources of variation (pollination period, family, family x pollination period and genotypes/family x pollination period) were not significant. The presence of modifier genes, which enhance 2n egg expressivity, would result in an increase of seed set after 2x x 4x crosses when specific parents are used.

The expected mean squares and narrow sense heritability estimate (h 2) for seed set after 2x x 4x crosses are shown in Table 5. Both additive (5.26) and dominance (7.23) variance were important for the expression of the trait. The h 2 was 0.24 and the average degree of dominance (a) was equal to 1.66 which might indicate overdominance. Stelly (8) suggested that five or more loci are involved in the control of 2x • 4x crossability. He also suggested that recessive alleles are responsible for 2n egg formation, while dominant alleles confer female fertility. This indicates that the value for degree of dominance, suggesting overdominance for seed set after 2x


TABLE 5.--Analysis of variance, expected mean squares and narrow-sense hen'tability for seedset after 2x* x 4x crosses during two pollination periods.

Source of Vat. d.f. M.S. E.M.S.

Environments 1 0.5610

Females 3 53.2473

Females x Environments 3 12.7638

Families/Females

Genotypes/Fam./Females

Env. x Fam./Females

Env. x Gen./Fam./Fem. Error

15 29.6559

178 34.7884**

15 13.6548 114 16.3213 703 21.5880

V e + 4.08 VEGFf + 33.58 VEF f + 1.71 VOF f + 2.35 V w + 147.13 V~f + 0.2918 Vf + 492.43 V~

V + 4.35 VEOFf + 30.99 VOF f + 6.75 Vcv f + 57.81 VFf + 125.75 Vrf + 240.95 Vf

V, + 4.01 VEGFf + 29.05 VEF f + 1.62 VOF f + 2.22 VFf + 115.20 VEt V + 4.34 VEVOf + 28.38 VZF f + 6.77 VGF f + 52.20 VF~ V + 3.47 V~CFf+ 0.58 VEF ~ + 5.04 Vov f V + 3.07 VEorf+ 16.92 VEF f

V e + 2.37 VEovf

Ve

h 2 = 0.2409

*haploid x species hybrids derived from crosses between homozygous recessive parents for omission of second division.

x 4x crosses, could be due to l inkage disequil ibrium, i.e. pseudo-overdominance .

Rtatrrent Selection to Increase 2n Egg Production and Seed Set After 2x x 4x Crosses

Den Nijs and Peloquin (5) found that crosses between clones with high 2n egg frequencies yielded a higher p ropor t ion of 2n egg producers with high frequencies of 2n egg produc t ion than crosses involving low 2n egg producers. T h e additive genetic var iance was still impor t an t for seed set after 2x x 4x crosses despite the fact that dominance var iance was greater than the former. Therefore , recur ren t selection can be appl ied to improve the f requency of 2n egg product ion in the diploid populat ion. However, the selected individuals, based on their phenotypic per fo rmance , should be included in progeny testing to identify them for use in the next cycle of recombinat ion .

Expec ted gains of selection us ing different n u m b e r of envi ronments , replications (fruits after pollination) and individuals per family are shown in Table 6. T h e intensity of selection used in the base popula t ion was 2 % (27 individuals with more than 10 seed per fruit were selected). At this selec-


TABLE 6.--Expected gain (G.S. 1) 9r intrapopulation recurrent selection for 2n egg production (seedset after 2x x 4x crosses) in 2x potato.

# Envi ronments (e)

# Fruits (r) #Genotypes (n) 1 2 3

i = 1% (k=2.665)

1 9 3.578 3.757 3.823 18 3.727 3.841 3.831 36 3.808 3.885 3.911

9 3.727 3.841 3.881 18 3.808 3.885 3.911 36 3.851 3.907 3.926

9 3.731 3.870 3.901 18 3.837 3.900 3.926 36 3.866 3.915 3.931

i = 2% (k=2.421)

1 9 3.251 3.413 3.473 18 3.386 3.489 3.526 36 3.460 3.529 3.553

9 3.386 3.489 3.526 18 3.460 3.529 3.553 36 3.499 3.543 3.562

9 3.435 3.515 3.544 18 3.486 3.543 3.562 36 3.512 3.556 3.572

i ffi 5% (k=2.063)

1 9 2.770 2.908 2.959 18 2.885 2.973 3.004 36 2.927 2.996 3.020

9 2.885 2.973 3.004 18 2.948 3.007 3.028 36 2.970 3.019 3.035

9 2.948 3.007 3.028 18 2.981 3.025 3.039 36 2.993 3.030 3.043

1G.S. ffi k x Va/[Vw/(ern ) + V / ( e r ) + V c/e + Vg] 1/2 where: . . . . P g .

k, selecuon mtenmty m s tandard umts Va, additive variance V , total genetic variance

g . .

W , genotype x e n w r o n m e n t variance g e . . .

V , between lndwlduals P . . . . .

Vw, wathm individuals

702 AMERICAN POTATO JOURNAL (Voi. 68

tion level the use of 18 individuals per family using 2 pollination periods (environments) and harvesting at least 2 fruits per genotype provide the basis for adequate gain from selection. Since the fruit set among these fam- flies was 0.40 during the first pollination period (high temperatures) and 0.66 in the second (low temperature), a m in imu m of five pollinations is required for each clone. The selected clones from crosses between haploid x species hybrids (Table 1) will constitute the first cycle of selection. They combine both high frequency of 2n egg production and good tuber appearance. They will be intercrossed in pairs to produce biparental progenies. The starting material will include at least five different species parents: S. bukasovii, S. chacoense, S. sparsipilum, and S. berthaultii x S. taro'ense, and five different haploid parents: H-556, H-551; W-730, and W-973 (both from W-231); and A-401 from Atlantic. Inbreeding should be avoided, since the final goal is to develop highly heterozygous elite 2x clones with a high frequency of 2n eggs and good tuber appearance.

Acknowledgmen t s

Paper No. 3120 from the Laboratory of Genetics. Research supported by the College of Agricultural and Life Sciences; International Potato Cen- ter; USDA-CRGO-88-37234 3619, and Frito-Lay, Inc.

Literature Cited

1. Falconer, D.S. 1989. Introduction to Quantitative Genetics. Third Edition. Longman. 2. Iwanaga, M. and S.J. Peloquin. 1979. Synaptic mutant affecting only megasporo-

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breeding for 2n egg production in haploid × species 2x potato hybrids

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